Geomorphology 234 (2015) 11–18
Contents lists available at ScienceDirect
Geomorphology
journal homepage: www.elsevier.com/locate/geomorph
Geodiversity, self-organization, and health of three-phase semi-arid
rangeland ecosystems, in the Israeli Negev
I. Stavi a,⁎, R. Shem-Tov a, M. Chocron b, H. Yizhaq a,c
a
b
c
Dead Sea & Arava Science Center, Ketura 88840, Israel
Department of Land of Israel Studies and Archaeology, Bar-Ilan University, Ramat Gan 52900, Israel
Department of Solar Energy and Environmental Physics, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, Midreshet Ben Gurion, Israel
a r t i c l e
i n f o
Article history:
Received 21 September 2014
Received in revised form 25 December 2014
Accepted 11 January 2015
Available online 17 January 2015
Keywords:
Bi-modal pattern
Ecosystem complexity
Herbaceous vegetation
Mesic vs. xeric conditions
Source–sink relations
Vegetative pattern
a b s t r a c t
Source–sink, two-phase mosaic-like ecosystems are widespread throughout the world's drylands. Such ecosystems are composed of woody vegetation patches and intershrub spaces and have been characterized as having
high flexibility and survivability. Recent studies from the semi-arid Negev drylands of Israel reported that livestock grazing has resulted in the modification of two-phase mosaic-like shrublands into three-phase mosaic
rangelands, with livestock trampling routes encompassing a separate, and the most degraded phase, while the
shrubs encompass the most improved phase. The objective of this study was, therefore, to reassess this theory
through the investigation of patch-scale (spatial scale of one to several decimeters) geodiversity and selforganization of these ecosystems. In terms of the effect of type of surface cover (microhabitat), the soil hygroscopic moisture content and stable aggregate content of the uppermost layer (0–5 cm depth) were significantly
affected by this factor, and revealed the highest, intermediate, and smallest values for the shrubby patches (3.06%
and 77%), intershrub spaces (2.81% and 68%), and the trampling routes (2.63% and 55%), respectively. An opposite
effect was recorded for the sand content, revealing 23.9%, 25.3%, and 26.0%, respectively. The clay dispersion
index was also significantly affected by microhabitat, and revealed a higher value for the trampling routes
(0.83) than for the intershrub spaces and shrub patches (0.37 for both). At the same time, other soil characteristics were not significantly affected by microhabitat. Overall, some differences were recorded between north- and
south-facing hillslopes, proposing somewhat better soil quality in the northern aspects. A conceptual model is
proposed, in which moderate livestock pressure increases ecosystem geodiversity at the patch scale, modifying
the ecosystem's self-organization to encompass a new (dynamic) equilibrium of a tri-modal pattern, and increasing ecosystem health. Also, a simple numerical simulation is proposed, modeling the effect of livestock trampling
routes on the redistribution of water at the patch scale, with the resultant modifications in distribution of vegetation cover. Yet, it is proposed that functioning of three-phase mosaic rangelands is more complex than previously suggested, encompassing several simultaneous effects, of which some may have offsetting impacts.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Extensive lands among the world's semi-arid regions have been reported as composed of two-phase mosaic-like ecosystems, containing
woody vegetation patches and bare or herbaceous vegetation-covered
ground in the intershrub spaces (Carter and O'Connor, 1991). Source–
sink spatial relations have been extensively reported to occur between
these two types of surface cover (microhabitats) (Merino-Martin et al.,
2012), where the intershrub spaces contribute runoff water and associated dissolved and suspended materials to the vegetation patches, which is
where these materials are accumulated and utilized for supporting
⁎ Corresponding author. Tel.: +972 8 630 6319; fax: +972 8 635 6634.
E-mail addresses: [email protected], [email protected] (I. Stavi).
http://dx.doi.org/10.1016/j.geomorph.2015.01.004
0169-555X/© 2015 Elsevier B.V. All rights reserved.
vegetation production (Imeson and Prinsen, 2004). High functioning capability, i.e., large capacity in retaining water and soil resources within an
ecosystem's boundaries while allowing only small leakage, characterizes
such ecosystems, which maintain their production capacity even during
consecutive drought years (Tongway and Ludwig, 2003). A wide range of
physical and biotic conditions have led to the formation of several patterns of vegetation patchiness, such as stripes, strands, stipples, and
others, efficiently exploiting the limited water and soil resources
(Ludwig et al., 1999). Such patterns of self-organization enable the survival of vegetation in drylands, where precipitation regimes could not
support full vegetation cover (Rietkerk et al., 2002; Borgogno et al.,
2009). Among other definitions of ecosystem health, one of the most important is its ability to support productivity, (self-)organization, and resilience, i.e., to carry on vegetation (and animal) growth, to sustain
diversity and interactions among its components, and to buffer
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I. Stavi et al. / Geomorphology 234 (2015) 11–18
perturbations, respectively (Rapport et al., 2013). Therefore, ecosystem
health should be the ultimate goal when discussing either naturally or
anthropogenically modified environments.
Geodiversity – defined as the natural range of geological, geomorphic, and soil features (Gray, 2005) – impacts biodiversity
(Jačková and Romportl, 2008) and, in addition, affects a range of ecosystem services and functions (Gray, 2004). The combination of biodiversity and geodiversity determines the overall natural diversity
(Cañadas and Flaño, 2007). Therefore, a holistic approach to the conservation of natural diversity can be achieved only if considering
both the living (biodiversity) and non-living (geodiversity) aspects
of the relevant ecosystems (Pemberton, 2007). Evaluation of
geodiversity must include the interpretation of processes and relationships (e.g., spatial redistribution of water and soil resources)
among its components (Gray, 2004).
Grazing lands cover more than 60 million km2 or 45% of the terrestrial surface of the globe (Reid et al., 2008). Livestock impact, including
the browsing of vegetation, excretion of feces and urine, and trampling
of soil, considerably affects the functioning and production capacity of
the rangeland ecosystems (Coughenour, 1991). A set of studies focusing
on the three-phase mosaic-like pattern was recently summarized by
Stavi et al. (2012), highlighting the effect of the non-even distribution
of livestock traffic on hillslopes on some characteristics of soil and vegetation in semi-arid rangeland ecosystems. Nevertheless, the impact of
livestock on patch-scale geodiversity, with a spatial scale of 1 dm to several decimeters, and its effects on the functioning of rangeland ecosystems has still remained greatly unknown. Therefore, the objective of
this study was to investigate the patch-scale geodiversity of these
rangelands, and to assess how it reflects on ecosystem functioning and
self-organization.
GPS apparatus. Mapping was based on delineation of the shrub patches'
perimeter and trampling routes' area. The routes were easily identified
by their exposed surface consisting of mechanical crusts, and by their
predominant, elongated lateral shape, transecting the hillslopes along
contours. Data were then digitized with ArcGIS software for the calculation of the cover percentage of each of these two types of microhabitats.
The cover percentage of the intershrub spaces excluding the trampling
routes was then calculated by subtracting the areas of shrubby patches
and of trampling routes from the total area of the plot (100 m2).
2.3. Soil sampling and infiltration testing
After mapping, soil samples from the uppermost soil layer (0–5 cm
depth) was obtained in five randomly selected spots of each of the
types of cover: shrub patches (of S. spinosum only), trampling routes,
and intershrub spaces excluding the trampling routes. To maintain consistency, the soil sample was taken from each of the shrub patches laterally rather than upslope or downslope, and on the western side of the
shrub center. The samples were carefully placed in a sealed plastic bag.
In addition, in proximity to each spot of the soil sampling, the infiltration capacity of water to soil under unsaturated conditions was also
tested. This was implemented by using a mini-disk infiltrometer
(Decagon®, USA) for 5 min (300 s) per spot.
Upon arrival to the laboratory, the soil samples were left to air-dry in
a well-ventilated space. Number of spots (n) for the soil samples as well
as for the infiltration tests was: 5 replicates × 3 microhabitats × 3
hillslopes × 2 aspects = 90.
2.4. Laboratory analyses
2. Materials and methods
2.1. Regional settings
The study was implemented at the Lehavim Demonstration Farm,
located in the northern semi-arid Negev (31° 20′ N, 34° 46′ E) of
Israel (Figs. 1 and 2). The area's lithology is chalk of the Eocene, with a
topography comprised of rolling hills. The mean altitude ranges between 350 and 500 m above sea level (Perevolotsky and Landau,
1988), and the soil is classified as Brown Rendzina (Dan and
Koyumdjisky, 1979). The predominant shrub species is Sarcopoterium
spinosum (L.) Spach, while Coridothymus capitatus (L.) Rchb.f. is also
prevalent in southern-facing hillslopes. The herbaceous vegetation consists of a range of grasses, forbs, and legumes. Mean daily temperatures
range between 11 °C in January and 25 °C in July; mean daily relative
humidity ranges between 67% and 50%, respectively; and mean annual
precipitation is approximately 300 mm (Bitan and Rubin, 1991). The
farm itself encompasses about 800 ha, where long-term livestock grazing has been implemented with a flock of approximately 800 head of
sheep and goats (Stavi et al., 2012).
2.2. Mapping of ground surface cover (microhabitats)
Fieldwork was conducted at the end of the dry season (September)
of 2013. Three pairs of north- and south-facing hillslopes were selected
for the study. This study scheme was implemented due to the prevailing
conditions of mesic and xeric habitats in northern and southern aspects,
respectively (see: Rigg, 1993), which are assumed to affect the ecosystem self-organization and functioning. A location was randomly selected along the backslope of each of these hillslopes and utilized for the
delineation of a 10 × 10 m plot. This plot size was chosen in order to ensure the capability of randomly selecting the sampling spots within
them. The plots were then mapped for their different types of cover – including shrub patches, livestock trampling routes, and the remainder of
the intershrub spaces – by using a high-resolution (10 cm precision)
Sub-samples of the soil were put in a drying oven (set to 105 °C,
for 24 h) to determine the hygroscopic moisture content. The main
soil samples were analyzed for texture (by the hydrometer method:
Bouyoucos, 1962), electrical conductivity (Richards, 1954), pH
(McLean, 1982), aggregate stability index (Herrick et al., 2001), stable aggregate content (by using an aggregate stability apparatus:
Eijkelkamp®, the Netherlands), and clay dispersion index. The latter
was determined through the positioning of an aggregate of 3 to
5 mm diameter in a Petri plate filled with distilled water, followed
by a visual observation of the extent of cloudiness (milkiness) after
10 min, and again, after 2 h. Dispersion index scores ranged from 0
for no cloudiness at all; 1 for slight cloudiness; 2 for moderate cloudiness; 3 for strong cloudiness, and 4 for complete cloudiness of the
aggregate's clays (adapted from: USDA-NRCS, EFH NOTICE 210-WI62). These soil characteristics were chosen due to their capacity in
representing the overall soil quality.
2.5. Statistical analysis
For analyzing the overall effect of the hillslope aspect, data processing was required. This included the normalizing of data according to the
relative cover percentage of each of the types of microhabitats on each
of the hillslopes.
Then, analysis of variance (ANOVA) was conducted with the GLM
(general linear model) procedure of SAS (SAS Institute, 1990). Factors
in the model were hillslope aspect (1 degree of freedom; df), block within hillslope aspect (3 df; error term for aspect), type of cover (2 df), and
the interaction hillslope aspect × type of cover (2 df). Statistically significant interactions were subjected to additional ANOVA with the SLICE
command of PROC GLM. Separation of means was implemented by
Tukey's HSD at a probability level of 0.05. Pearson correlation coefficients were computed to assess the relations between each pair of
variables.
I. Stavi et al. / Geomorphology 234 (2015) 11–18
13
Fig. 1. Map of the study site in Israel.
3. Results and discussion
3.1. Ground surface cover (microhabitats)
Concordant with previous studies (summarized in Stavi et al., 2012),
the hillslope's surface cover was found to be of a patchy nature. The
intershrub spaces were found to have the greatest mean cover
(61.1 ± 6.2%), shrub patches, an intermediate cover (28.1 ± 7.1%),
and livestock trampling routes, the smallest cover (10.8 ± 1.9%). The effect of hillslope aspect on the mean cover percentage of the different microhabitats was considerable, with shrubs having much greater mean
cover percentage in the northern facing hillslopes, and routes having
considerably greater mean cover percentage in the southern facing
hillslopes (Fig. 3). Two GIS-based maps of representative northernand southern-facing hillslopes are shown in Fig. 4.
It is noteworthy to mention that the overall mean cover percentage
of trampling routes observed in this study was only about a half of that
reported in a recent study (Stavi et al., 2012: ~21%) which was implemented in the same region. This could be attributed to the mapping of
pairs of hillslopes in this study different to those which were utilized
in Stavi et al. (2012), suggesting wide heterogeneity among hillslopes
in the study region.
3.2. Hillslope effect
The greater mean shrub cover on the northern-facing hillslopes corresponds with previous studies which showed that compared with
south-facing hillslopes, the smaller loss of soil moisture through
evaporation on north-facing hillslopes (Shoshany, 2002) results in the
formation of mesic conditions (Rigg, 1993), increasing vegetation
growth and augmenting net primary productivity (NPP) (Bochet and
García-Fayos, 2004). The greater NPP in the northern hillslopes is expected to improve the physical and hydraulic characteristics of the surface soil (Archer et al., 2002), further increasing the retention of water
and soil resources within the ecosystem boundaries (Andreu et al.,
2001). This concept accords with the study results, revealing significantly greater means of normalized aggregate stability index, stable aggregate content, and hygroscopic moisture content in the north- than in
the south-facing aspects. In addition, the significantly greater mean normalized soil pH in the southern, rather than in the northern hillslopes,
demonstrates the lower limitation of vegetation productivity due to
soil alkalinity in the north-facing aspects than that in the south. Yet,
the mean normalized pH level under both of the northern and southern
aspects was only slightly alkaline and, presumably, had no impact on
the ecosystem production capacity. At the same time, no significant effect of hillslope aspect was recorded for any of the mean normalized
values of soil electrical conductivity, clay dispersion index, and unsaturated infiltration capacity (Table 1). However, the mean normalized soil
texture was considerably impacted by the hillslope aspect. This was revealed by the significantly greater silt content and significantly smaller
sand content in the northern hillslopes than those in the southern
hillslopes (Table 2). Overall, the smaller silt content and larger sand
content of the surface soil indicate the sorting – through erosional processes – of the finer fractions (see: Zhang et al., 2014), and suggest that
the southern aspects are more susceptible to hillslope-scale erosional
processes than the northern aspects (e.g., Istanbulluoglu et al., 2008).
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I. Stavi et al. / Geomorphology 234 (2015) 11–18
Fig. 2. Characteristic landscape of the study region.
3.3. Microhabitat effect
The highly significant effect of type of cover on the mean of each of
the stable aggregate content and hygroscopic moisture content clearly
demonstrates the existence of three different microhabitats in this
type of ecosystem. The greatest, intermediate, and smallest values of
these variables for the shrub patches, intershrub spaces, and trampling
routes, respectively (Table 3), accord with Stavi et al. (2012), who reported the same trend of overall quality of soil. These findings are attributed to the source–sink relations, where the trampling routes act as
optimal source areas and the shrubby patches act as optimal sinks,
while the intershrub spaces (excluding routes) lay in between these
two extremes. The greatest and smallest mean contents of sand fraction
under the trampling routes and shrubby patches, respectively, as well as
the (though not significantly) smallest clay content in the routes
(Table 4), demonstrate the sorting of the finer fractions off the routes.
Therefore, it can be assumed that suspended fine mineral materials, dissolved materials, and floating organic materials that are generated in
the trampling routes are accumulated in the shrubby patches and to a
smaller extent also in the intershrub areas. The latter two microhabitats
experience the improvement in soil structure formation and aggregate
Fig. 3. Mean cover (%) of shrub patches, intershrub spaces, and trampling routes, by hillslope aspect.
stability, resulting in an increase in the soil hygroscopic moisture content. At the same time, the absence of vegetation in the trampling
routes, coupled with their smooth surface, negates the retention of
water and deposition of fine mineral material and organic material on
their surface, and prevents the development of well-structured soil in
this microhabitat. Over the long run, the reoccurrence of intense livestock traffic on the trampling routes enables these processes to be
self-sustaining. These results strengthen the recently proposed concept
(summarized in Stavi et al., 2012), according to which the consideration
of such ecosystems as two-phase mosaics is an over-simplification.
The mean clay dispersion index, despite being significantly affected
by microhabitat, was similar between the shrub patches and intershrub
spaces, demonstrating the complexity of the functioning of such threephase mosaic-like geo-ecosystems. One way or another, the significantly and considerably smaller clay dispersion index under these two microhabitats than that under the trampling routes exemplifies the
inferior physical quality of soil under the latter (Table 3). Regardless,
these results are in accordance with the concept of ‘fertility islands’
(Garner and Steinberger, 1989), where vegetative patches are claimed
to operate as sinks of water runoff that is generated in the intershrub
spaces (Saco et al., 2007). According to this concept, along the temporal
axis, the soil quality of such mosaic-like ecosystems is getting improved
in the vegetative patches and degraded in the intershrub spaces
(Vásquez-Méndez et al., 2010).
At the same time, the effect of type of cover on mean aggregate stability index, pH, and unsaturated water infiltration capacity was not significant (Table 3). To some extent, the absence of significant effect of
microhabitat on the unsaturated infiltration capacity may be attributed
to the recorded very high variability for this soil feature, which by itself,
could be attributed to a finer-scale heterogeneity of the surface soil.
Also, despite the considerable differences in the mean soil electrical conductivity among the various microhabitats, the high variability of this
soil characteristic negated a significant effect. Yet, the much greater
electrical conductivity of the soil under the trampling routes than that
under the shrub patches and intershrub spaces could be attributed to
the absence of a fine root system in this microhabitat, decreasing salt
leaching from the uppermost soil layer.
The effect of the interaction type of cover × hillslope aspect was significant only for the soil's mean silt and sand contents. The mean silt
content was significantly different between the northern and southern
hillslopes only for the trampling routes, being greater in the more
I. Stavi et al. / Geomorphology 234 (2015) 11–18
15
Fig. 4. GIS maps of the plot in a representative (the “FOREST”) northern (a) and southern (b) aspects.
mesic aspects than in the more xeric aspects. An opposite effect was recorded for the sand content under each of the intershrub spaces and
trampling routes (Table 5).
3.4. General data integration and knowledge gaps
Over recent years, poor maintenance of the fences surrounding the
livestock-exclusion plots across the Lehavim Demonstration Farm negated the investigation of the actual effect of livestock grazing on the rangeland geodiversity. Regardless, the obvious effect of the grazing animals on
the formation and persistence of the trampling routes highlighted the impact of livestock in increasing patch-scale geodiversity. Also, obtaining
undisturbed soil cores was impossible because of the extremely high content of rock fragments in the soil, imposing technical difficulties in investigating the effect of hillslope aspect and microhabitat (type of surface
cover) on the soil's available moisture capacity. Yet, the results of some
of the studied soil properties, such as the stable aggregate content, hygroscopic moisture content, aggregate stability index, clay dispersion index,
pH, and texture, highlighted the considerable effect of hillslope aspect
and microhabitat on soil quality. Overall, despite some discrepancies,
the obtained results affirm the previously proposed concept, suggesting
that livestock trampling routes constitute a separate microhabitat,
which causes the two-phase mosaic-like patterns to function as threephase ecosystems (Stavi et al., 2012).
The concept of natural diversity encompasses two components:
(1) the number of different types of objects (e.g., biological species
and soil types) in a mixture or a sample, and (2) the relative size or
number of each type of object, as well as its distribution among the
other objects (Ibáñez et al., 2012). At the same time, two important concepts for the quantification of diversity are: (a) whether the specific
groups are different enough to be considered separate types of objects,
and (b) whether the objects in each specific group are similar enough to
be considered the same type (Huston, 1994). According to these concepts, the considerable cover of trampling routes (almost 11%), their
spatial reoccurrence, and the remarkable differences between them
and the other types of surface cover, make them an important determinant of the geo-ecosystem diversity. Regardless, some of the obtained
results suggest no clear difference between the shrubby patches and
the intershrub spaces excluding the trampling routes. Also, the absence
of a strong correlation (r N 0.50) between any pair of the studied soil
characteristics further demonstrated the geo-ecosystem's complex nature, with the presumably simultaneous impacts of offsetting mechanisms between them.
Along the soil quality continuum, the shrubby patches and trampling
routes represent the maximum and minimum extremes, respectively,
with the intershrub spaces lying somewhere between those extremes.
The spatial relations among the different microhabitats are proposed
to form positive feedbacks, which strengthen the existing state and conditions in each of them (Fig. 5). For example, high-intensity trampling in
the routes is assumed to grind and shear the uppermost soil layer in this
microhabitat. The ground and sheared mineral material becomes available for suspension in water and to flow downslope with the runoff,
where it is deposited either in the shrubby patches or intershrub spaces
excluding routes. Reoccurrence of these processes depletes the fine
Table 1
Effect of hillside aspect on the soil's unsaturated infiltration rate (cm s−1); aggregate stability index (1 through 6: the higher the index, the greater the stability); stable aggregate content
(%); clay dispersion index; hygroscopic moisture content (%); electrical conductivity (μS), and pH.
P value
North
South
Infiltration rate
Stability index
Stable aggregate
Clay dispersion index
Hygroscopic moisture
Electrical conductivity
pH
0.1442
0.00059a (0.00006)
0.00046a (0.00017)
0.0259
5.77a (0.10)
5.40b (0.14)
0.0001
79.0a (2.0)
58.3b (3.4)
0.185
0.32a (0.09)
0.51a (0.11)
0.0001
3.07a (0.10)
2.68b (0.06)
0.9424
702.1a (36.0)
706.0a (47.4)
0.0101
7.67b (0.06)
7.88a (0.07)
Notes: Means within the same column followed by a different letter differ at the 0.05 probability level according to Tukey's HSD. Numbers within parentheses are standard error (SE) of the
means.
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I. Stavi et al. / Geomorphology 234 (2015) 11–18
Table 2
Effect of hillside aspect on soil contents of clay, silt, and sand (in %).
P value
North
South
Table 4
Effect of microhabitat (type of cover) on the soil contents of clay, silt, and sand (in %).
Clay
Silt
Sand
0.9023
27.3a (0.9)
27.2a (0.6)
0.0006
49.6a (0.9)
45.8b (0.7)
0.0001
23.1b (0.6)
27.0a (0.5)
Notes: Means within the same column followed by a different letter differ at the 0.05 probability level according to Tukey's HSD. Numbers within parentheses are standard error
(SE) of the means.
mineral material in the route's surface and increases its accumulation in
the remainder of the microhabitats. At the same time, the surface roughness in the intershrub spaces induced by the herbaceous vegetation and
rock fragment cover, and shrubby patches increases the sinking capacity
of water, mineral material, and coarse organic matter in these microhabitats. These processes stimulate vegetation growth, accelerating
the retention of the self- (on-site originated) and imported- (off-site
originated) resources in the vegetated microhabitats, and negating the
restoration of soil quality and production capacity in the trampling
routes. Reoccurrence of concentrated livestock traffic in the routes, as
opposed to the sporadic trampling in the vegetated microhabitats, further accelerates these feedbacks.
A study implemented in a protected landscape area in the Czech
Republic reported that geodiversity, including (macro-)topographic
variability and relief heterogeneity, positively affected plant taxon richness (Jačková and Romportl, 2008). Recently, a geodiversity index was
developed, enabling comparison among different sites. This index considered the number of physical (including geologic, geomorphic, hydrologic, and pedogenic) elements involved in the studied site, the surface
area of the studied site (to a km2 scale), and the roughness of the unit.
Yet, this index could not be utilized for smaller-sized aerial units, and
is not applicable for determining smaller-scale geodiversity (Cañadas
and Flaño, 2007). Moreover, even though geodiversity studies generally
consider soils, only rarely do they relate their specific features to
geodiversity (Ibáñez et al., 2012), mainly focusing on the background
data, such as geology and topography. In our study region, surface heterogeneity (or diversity) was previously suggested to be reflected
through the sharpening of the hillslopes' micro-topographic (to a scale
of several decimeters) step-like profile. This effect was proposed to be
associated with the livestock trampling routes, increasing the discontinuity of geomorphic processes, and affecting redistribution of water
and soil resources at the patch- and hillslope-scales (Stavi et al.,
2012). It therefore seems that while geological background is prominent in determining geodiversity at the macro, landscape scale, the effect of livestock grazing is particularly considerable at the patch scale.
For summarizing the impact of livestock grazing on the rangeland
ecosystems, we propose a conceptual model which describes the relationship between the stocking rate and each of the patch-scale
geodiversity, the ecosystem self-organization, and the ecosystem health
(Fig. 6). A long-term moderate stocking rate increases the patch scale
geodiversity, from a two-phase into a three-phase geo-ecosystem, and
modifies the ecosystem's self-organization — from a bi-modal
(e.g., Rietkerk et al., 2002) to a tri-modal pattern. It is suggested that
this new state of (dynamic) equilibrium increases the redistribution of
water and soil resources at the patch scale. Regardless, the greater
P value
Shrub patches
Intershrub spaces
Trampling routes
Clay
Silt
Sand
0.0814
27.2a (0.6)
28.8a (0.6)
25.9a (1.3)
0.0846
48.8a (0.8)
46.0a (0.7)
48.2a (1.5)
0.0522
23.9b (0.8)
25.3ab (0.8)
26.0a (0.7)
Notes: Means within the same column followed by a different letter differ at the 0.05 probability level according to Tukey's HSD. Numbers within parentheses are standard error
(SE) of the means.
geodiversity is proposed to support a wider range of biological species
(biodiversity) and activities, improving ecosystem health (see:
Rapport et al., 2013). At the same time, it could be assumed that the impact of livestock grazing on geodiversity is directly dependent on the
stocking rate. In this regard, it is assumed that an excessively high livestock rate (over-grazing) diminishes geodiversity and modifies the ecosystem into a one-phase form, being either exposed of vegetation
(Gamoun et al., 2010) or fully covered with woody vegetation which
is not edible for grazing animals (Schlesinger et al., 1990). Consequently, the ecosystem's self organization is lost and the landform functioning
is modified. In the event of the one-phase form consisting of only exposed surface, the ecosystem functioning becomes considerably degraded, as the leaking of water and soil resources off the ecosystem
boundaries becomes the most prominent process. As opposed to that,
if the one-phase form consists of full cover of woody vegetation, the
retaining capacity of water and soil resources within the ecosystem
boundaries becomes considerably large, augmenting the ecosystem
functioning. One way or another, being either exposed or fully covered
with woody vegetation, the smaller geo- and bio-diversity results in the
degradation of ecosystem health. Also, as shown by Schlesinger et al.
(1990), the economic usability of rangelands that become fully covered
with woody vegetation is lost. Regardless, in the event of edible woody
vegetation, a new state of a two-phase ecosystem may be formed,
consisting of shrubby patches and exposed intershrub spaces. Despite
possibly being efficient in resource conservation, the species diversity
and ecosystem health are expected to become degraded under this
new two-phase form.
Unlike biodiversity evaluation, standardized methods for evaluating
geodiversity have yet to be established (Jačková and Romportl, 2008).
This study revealed that such methods are specifically absent for the assessment of small-scale geodiversity. Particularly, for better understanding the impact of livestock on patch-scale geodiversity and
ecosystem self-organization of mosaic-like patterned rangelands, additional studies are needed to examine the actual effects of different
stocking rates. This could be implemented by using livestock enclosures
and applying several grazing regimes, comparing them to long-term
grazing exclusion plots as a reference treatment. In addition, so far,
geodiversity studies are almost absent in mathematical models of vegetation patterns (e.g., Borgogno et al., 2009).
One possible way to quantify the three-phase mosaics is by modifying the model proposed by Kéfi et al. (2010), which described the formation of vegetation patterns in water-limited environments. In this
model, the pattern-forming feedback is based on the infiltration contrast between vegetated and bare-soil domains, which is dictated by
Table 3
Effect of microhabitat (type of cover) on the soil's infiltration rate (cm s−1); aggregate stability index (1 through 6: the higher the index, the greater the stability); stable aggregate content
(%); clay dispersion index, hygroscopic moisture content (%); electrical conductivity (μS), and pH.
P value
Shrub patches
Intershrub spaces
Trampling routes
Infiltration rate
Stability index
Stable aggregate
Clay dispersion index
Hygroscopic moisture
Electrical conductivity
pH
0.3327
0.00059a (0.00017)
0.00064a (0.00023)
0.00052a (0.00015)
0.109
5.80a (0.07)
5.53a (0.12)
5.43a (0.16)
0.0001
77a (0.02)
68b (0.03)
55c (0.02)
0.0177
0.37b (0.11)
0.37b (0.10)
0.83a (0.17)
0.0001
3.06a (0.08)
2.81b (0.07)
2.63c (0.06)
0.2583
701.3a (23.9)
682.5a (36.8)
783.8a (72.8)
0.7856
7.75a (0.06)
7.79a (0.05)
7.76a (0.06)
Notes: Means within the same column followed by a different letter differ at the 0.05 probability level according to Tukey's HSD. Numbers within parentheses are standard error (SE) of the
means.
I. Stavi et al. / Geomorphology 234 (2015) 11–18
17
Table 5
Effect of the interaction between hillside aspect and microhabitat (type of cover) on the
soil's silt and sand contents (in %).
P value
North aspect × shrub patches
North aspect × intershrub spaces
North aspect × trampling routes
South aspect × shrub patches
South aspect × intershrub spaces
South aspect × trampling routes
Silt
Sand
0.0017
48.1abc (1.1)
48.4abc (0.9)
52.4a (2.3)
49.5ab (1.2)
43.6c (0.5)
44.1bc (1.3)
0.0059
23.5b (1.4)
22.6b (1.1)
23.1b (0.7)
24.4b (0.8)
28.0a (0.5)
28.7a (0.6)
Notes: Means within the same column followed by a different letter differ at the 0.05 probability level according to Tukey's HSD. Numbers within parentheses are standard error
(SE) of the means.
the parameter α that stands for maximum soil water infiltration (see:
Kéfi et al., 2010; Yizhaq et al., 2014). The concept is to define the trampling routes with lower α values than the background. Figs. 7 and 8
show the vegetation biomass and the soil water distribution, respectively, for a domain with five trampling routes and under four different α
values. The greater the α value, the larger the effect of trampling routes.
The three-phase mosaic can be easily observed for the soil water distribution, where the lowest values exist in the trampling routes, intermediate values in the bare soil, and the highest values in the vegetation
patches. The trampling routes act as a strong source for the water, increasing its redistribution, and augmenting the provision of water for
the nearby vegetation patches. Yet, it should be emphasized that this
model is of a simple nature, and has to be thoroughly elaborated in
order to more precisely describe the role of trampling routes in vegetation pattern formation. Regardless, future efforts should model the relations among patch-scale geodiversity, self-organization, and ecosystem
health in water-limited environments. Moreover, for wider verification
of the concept of three-phase mosaics, similar studies have to be implemented in additional semi-arid rangelands around the world.
Fig. 6. Conceptual model of the effects of livestock rate on the rangeland ecosystems'
geodiversity, self-organization, and health: low livestock rate has no effect on the existing
two-phase system, which supports the bi-modal self-organization, characterized by a fair
state of health; moderate livestock rate modifies the geodiversity to a three-phase pattern,
resulting in the formation of a tri-modal self-organization, which is characterized by a high
state of health; high livestock rate eliminates the vegetation cover, resulting in the loss of
self-organization and the state of poor ecosystem health. *Note: in specific occasions, a full
cover of inedible woody vegetation could be formed, but yet, the overall health of such
ecosystems would be rather low.
in the remainder of the intershrub spaces and in the shrub patches,
which act as sink of these resources. Concordant with the modifications
in the physical characteristics of the routes, their chemical and biochemical characteristics are also modified. The resultant increased
geodiversity of the hillslopes considerably regulates the spatial distribution of vegetation and modifies the functioning of the rangeland geoecosystem. Unlike the common perception of bi-modal self-organization
patterns, such rangelands encompass tri-modal patterns, resulting in
greater ecosystem health. Yet, compared to previous studies, the present
study suggests that even the consideration of such shrublands as tri-
4. Conclusions
This study highlighted the role of livestock trampling routes in determining geodiversity at the patch scale of semi-arid rangelands. The intensive trampling along certain trails modifies their physical characteristics,
making them optimal source areas of resources. These are accumulated
Fig. 7. Numerical simulation of biomass density (g m− 2) in a unit area with five trampling routes, applied to the model by Kéfi et al. (2010) and by Yizhaq et al. (2014).
Panels a, b, c, and d correspond to different values of α in the trampling routes
which is the maximum soil water infiltration, α = {0.2, 0.18, 0.12, 0.06 d− 1 } respectively. The precipitation rate (R) is 1.56 mm d − 1 and the spatial domain is
50 × 50 m. All other parameters (see Kéfi et al., 2010 for the model details) are identical
in all panels and are given by: c ¼ 10; gmax ¼ 0:05 mm−1 m−2 ; k1 ¼ 5 mm; d ¼ 0:25 d
Fig. 5. Soil quality continuum (in grey) and feedback relations (in black) at the patch scale,
by type of surface cover.
; k2 ¼ 5 gr m
m2 d
−1
−2
; W 0 ¼ 0:2; r w ¼ 0:2 d
−1
; Ds ¼ 25 m2 d
.
−1
; i0 ¼ 0:06 d
−1
2 −1
; Dp ¼ 0:005 m d
−1
; Dw ¼ 0:1
18
I. Stavi et al. / Geomorphology 234 (2015) 11–18
Fig. 8. Soil water distribution (in mm) for the simulation described in Fig. 7. Panels a, b, c,
and d correspond to different values of α in the trampling routes which is the maximum
soil water infiltration, α = {0.2, 0.18, 0.12, 0.06 d−1} respectively. The three-phase pattern is clearly visible for the lower values of alfa: the soil–water content is minimal at
the trampling routes, maximal under the vegetation patches, and intermediate at the
bare soil.
modal ecosystems may be an over-simplification, demonstrating the inherent complexity of the functioning of semi-arid rangelands.
Acknowledgments
The authors are grateful for ICA in Israel (the JCA Charitable Foundation), for participating in funding this study. The authors kindly acknowledge Professor Xulong Wang and two additional anonymous
reviewers for their very helpful comments on a previous version of
this manuscript.
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